The present invention relates generally to quantitative ionizing radiation detectors and morek particularly to a tritium monitoring system.
One of the considerations in the operation of heavy-water moderated reactors is the measurement of the radioisotope, tritium. Further, with tritium-fueled fusion reactors nearing reality and research in this area growing enormously with time, tritium monitoring is becoming increasingly important to those whose responsibility includes human hazard prevention and detection. The Kanne chamber (see, e.g., J. E. Hoy, Health Physics 6, 203 (1961)) has been used for more than twenty years to monitor radioactive gases. It is especially suitable for monitoring weak beta particle emitters since their range in air is short when compared with the overall dimensions of such chambers. Other more energetic gaseous radioisotopes are detected with reduced efficiency since less energy is deposited in the chamber itself and more lost to the surrounding walls. A conventional embodiment of this device consists of three concentric cylinders, the inner and outer of these being held at ground potential while the intermediate cylinder is operated at approximately 200 V. The region between the outer and intermediate cylinders serves as an ion trap. This allows the device to detect tritium beta particle emission from gas actually present within and passing through the ion chamber which comprises the intermediate cylinder and inner cylinder, free from previous ionizing events. Decomposition of radionuclides present in the ion chamber is detected by means of a current developed between the inner and intermediate electrodes as a result of migration of charged species formed when the energy of the emitted particles is deposited in the air surrounding the electrodes. Meaningful calibration and detection can thereby be achieved. Typically, 51.6 liters is the active volume since high sensitivity is a function of volume for this type of device. When the Kanne device is used to detect tritium in the ambient breathing air, contamination is not a problem. However, when exposed to high concentrations of radioactive gases such as HTO, for example, or gases contaminated with tritiated oil, a buildup of background activity may occur which significantly reduces the sensitivity of the chamber to low tritium concentrations. Further, electronic compensation for a large background is difficult and often unreliable. Restoration of the uncontaminated sensitivity may require procedures ranging from simple purging of the chamber for several hours with clean air to more drastic heating or disassembly and cleaning with their more significant down time. Occasionally, a badly contaminated unit may have to be discarded.
In order to reduce the problem of contamination and improve the ability to decontaminate the chamber when that occasion arises, the apparatus of the instant invention is designed to operate with significantly diminished contamination sensitive area; that is, the surface area which when contaminated destroys the high sensitivity of the apparatus is substantially reduced without deleterious effects on its overall operation. This is achieved by replacing the solid high voltage cylinder of the conventional Kanne chamber by an open, wire cylinder of comparable dimensions, and the usual solid, cylindrical central collector electrode by a much smaller surface area rod. The instant design allows most required decontamination to be accomplished with the chamber assembled, and permits existing Kanne chambers to be trivially retrofitted with the improved electrodes. Since our design does not include an internal deionizer, an external deionizer is attached.
At the present time, the principle reference relating to the instant invention is the Hoy article, supra, which describes the construction and operation of conventional Kanne chambers. The most important difficulty with this very useful device, that of possible contamination with subsequent loss of sensitivity, has been briefly discussed herein above. Hoy teaches that few contamination problems exist if the air to be analyzed is filtered, but that residual activity may build up when high concentrations of radioisotopes are passed through the chamber. In this event, Hoy has found that several hours of dry air purging and heating may remove many gaseous contaminants, but disassembly and cleaning for more severe contamination is sometimes necessary. The modifications taught by our apparatus take advantage of all of the positive aspects of the Kanne design, while essentially eliminating the contamination problem. Further, said modifications can be easily adapted to existing Kanne units.